PAE polymers are well-known as wet strength additives in papermaking processes. Technology for using these PAE polymers as curing agent components for protein or lignin-based adhesives is known (US application 2005/0282988). The use of the PAE's with proteins, such as soy flour, soy protein isolate or lignin as adhesives for lignocellulosic is known. (U.S. Pat. No. 7,252,735).
A current limitation of this type of adhesive is often the desirable solids content. In addition PAE resins of high solids content have previously been limited by high viscosity and poor aging stability. There remains in the use of PAE-containing adhesives a need for high solids stable, PAE resins to enable high solids adhesive formulation, which under normal process conditions for a given application, produce strength and adhesive properties in composite samples that are nearly equal to or better than the current low solids PAE based formualtions.
PAE resins with low molecular weight are known (US2007895122A). Such materials result in lower viscosities. However, to obtain low viscosity at a higher solids and retain aging stability requires additional knowledge as discovered in the current invention.
U.S. Pat. No. 5,567,798 (column 6 lines 60-61) teaches that “to prolong shelf life, a residual solids content of 30% or less is preferred.” By contrast the PAE resins of the current invention surprisingly are stable to solids of 40% by weight or greater.
U.S. Pat. Nos. 3,197,427, 4,853,431, 6,908,983 B2, and 7291695 B2 teach a low pH, is needed for stability. In the first the pH range is 2.0 to 3.0, in the second it is below 4.2 and most preferably 3.2 to 3.4, in the third it is below 3.3, and in the forth it is below 3.0. Contrary to these and surprisingly PAE resins of higher pH were stable in the current invention. This is even far more surprising considering the high solids of the PAE resins of the current invention, and even more surprising considering the current resins not only have viscosity stability but also stability of high azetidinium group functionality, which as will be noted later, is important.
US patent application 2005/0261404 A1 (paragraph 32 and 33) teaches the possibility of obtaining a high solids suspension of protein material even up to 50%. However, those familiar with protein containing materials know that not all protein sources can be used at such high solids. For example undenatured soy flour when mixed with water can be used up to about 35% solids. Above 35% solids, the viscosity rapidly increases until the material can no longer be stirred, readily mixed or pumped. The viscosity, even with the addition of a viscosity modifier such as sodium bisulfite is high. With a 1% level of sodium bisulfite, on a basis of dry soy flour weight, the viscosity of an aqueous mixture at 30% solids is about 400 cps, at 33% solids it is about 1800 cps, and at 36% solids it is about 20,000 cps. The viscosity continues to increase exponentially with solids. One can not readily mix or pump a soy flour/water mixture above about 36% solids. Therefore, there is a need as shown in the current invention to obtain higher solids through increasing the solids of the PAE component.
US patent application 2005/0261404 A1 (paragraph 21) also limits the formulations of the patent to soy protein free from urease. This is not a necessary requirement for an adhesive based on the combination of a protein source with a PAE resin.
Higher solids adhesives, enabled by higher solids PAE resins, are required in certain applications for successful use and or composite manufacturing. For example in a typical particle board manufacturing process about 93 parts of wood are combined with 7 parts of a urea formaldehyde based adhesive and a typical urea formaldehyde (UF) resin has a solids content of 65%. Those familiar in the art know that employing of substantially lower solids under normal process conditions of a particle board mill will result in a failure to properly internally bond the particle board resulting in defect boards. (see Modern Particleboard & Dry-Process Fiberboard Manufacturing by T. M. Maloney, 1977 Miller Freeman Publ.).
Although UF resins are very strong, fast curing, and reasonably easy to use, these resins lack hydrolytic stability along the polymer backbone. This causes significant amounts of free formaldehyde to be released from the finished products (and ultimately, inhaled by the occupants within the home). There have been several legislative actions to push for the removal of these resins from interior home applications (Health and Safety Code Title 17 California Code of Regulations Sec. 93120-93120.12).
Replacement or extension of a UF adhesive is greatly desirable because of the negative environmental impact of formaldehyde. PAE and soy adhesives, such as a PAE/soy flour mixture have previously been made commercially with PAE resins of 20 to 30% solids. The resulting adhesive can not achieve the 65% solids of a UF resin when significant levels of the PAE and soy flour are used under normal conditions. That is high solids has only been obtained by the addition of high levels of low molecular weight components such as urea and glycerol (US patent application 2009/0098387). Useable adhesives with viscosities below 5000 cps such as with a UF resin, in which at least 40 and preferably greater than 50% of the solids composition is a combination of PAE and a protein source such as soy flour, have not previously been realized.